It has been known for over 20 years in the field of impact modifiers for poly(vinyl chloride) (PVC) that larger particle rubber cores sometimes are desirable for improved impact in core/shell impact-modified PVC. Especially with butadiene-based cores, it is a lengthy process to enlarge the particle size of the core by further polymerization of more butadiene (or copolymer of butadiene with styrene) in the presence of the core under conditions where no new particles are formed (the "grow-out" process), although such processes are readily accomplished with acrylic rubber cores. An alternate method is to agglomerate the rubber particles by what is essentially a controlled partial coagulation by adjustment of pH, salt content, and the like, so as to give a cluster of particles, which then are used as a substrate onto which is polymerized one or more hard stages or shells. The so-agglomerated rubber behaves in imparting impact and in optical microscopy in a manner similar to grow-out particles of similar particle size.
However, there are also reasons in impact modification of PVC to prepare core/shell impact modifiers wherein the rubber is of relatively low particle size. These reasons include systems, i.e., processed PVC compounds where high clarity is desired, or where it is desired to pigment the system with avoidance of a washed-out or pearlescent appearance. These reasons further include the field of acrylic-based impact modifiers, where surprisingly particle sizes in the range of 50 to 100 nm are effective modifiers. However, such systems involving small particle sizes, especially if high in rubber content, are difficult to prepare at high solids, and are difficult to isolate by means such as coagulation or spray-drying. Post-addition approaches or co-spray-drying approaches have been utilized, but often dilute the efficacy of the impact modifier.
Further, there are other thermoplastics where smaller particle rubbers are preferred for a particular balance of impact properties, processing, and surface appearance, such as those polymers which do not craze on impact, such as polyesters.
Thus the art has sought a means for preparing core/shell impact modifiers for PVC and other thermoplastics wherein the rubbery core content is high, wherein the particle size of the modifier is small, wherein the material can conveniently and inexpensively be isolated in powdery form, and wherein the polymer particles readily redisperse to the initial small particle size.